Time alignment is the process of controlling the time of TDMA time slot burst transmission from the mobile station by advancing or retarding the mobile transmit burst so that it arrives at the base station receiver in a proper time relationship to other time slot burst transmissions. Time alignment applies to TDMA channels in the direction from mobile to base station. An error in time alignment may cause the arrival of signals from two different mobile transmitters simultaneously at the base station receiver. This in turn causes errors in both signals. This overlap can occur at the beginning or end of a time slot. Upon detecting an overlap condition, the base station sends an appropriate time alignment message to the mobile station using the appropriate forward control signaling channel.
According to the Section of the IS-54, December 1989, defining system access, the mobile station receives an initial traffic channel designation message and seizes a traffic channel. The mobile station transmits a shortened time slot burst at a predetermined time until it receives a time alignment message from the base station or the mobile station is directed to stop transmission due to other considerations. The burst sent by the mobile is artificially shortened in order to reduce the chances for signal overlap. If the mobile station receives a time alignment message, it adjusts its transmission timing and transmits during the next available slot a time-aligned, full duration slot burst.
Thus, the time alignment parameter is initially reset to zero and adjusted to an optimal value in typically about one to four seconds. By the time the alignment parameter is adjusted, a number of bit errors may have already occurred due to burst overlap.
At handoff of a mobile station to a new base station, the distance between the mobile station and the (currently active) base station which coordinates communications with that mobile changes abruptly, requiring a change of the absolute time alignment offset parameter in the mobile station. At handoff, the time alignment offset parameter is either reset to zero which is the value associated with the mobile station being near the base station, or the old time alignment offset parameter is not modified. Then, the time alignment parameter is adjusted by measuring the time when a burst is received at the new base station, based on the sync pattern in the burst, and a change of the time alignment offset parameter is ordered responsive to this measurement. This change typically takes about one to four seconds. During this delay, there is an increased probability of disturbing other time slots due to burst overlapping.